Can Cameron win a Leveson vote? The Commons arithmetic untangled

On Monday evening the division bell will ring in parliament and MPs will vote for the future of Britain's press. It won't be clear, from the tweets of MPs emerging from the voting lobbies, which side is going to win. That's because this Lib-Lab vs Cameron standoff puts the Commons on a knife-edge. There's a reason the prime minister voluntarily used the phrase "hung parliament" in his press conference in Downing Street this morning.

The Conservatives currently have 303 MPs, down three from their 2010 general election total. That contrasts with Labour's 255. With Nick Clegg's party joining Ed Miliband's - they are also steady on 57 following their Eastleigh by-election hold - the Lib-Lab alliance tots up 312 votes, nine ahead of the Tories. Even if it were a straightforward standoff between the three main parties, this would be a close one.

And then there's the minority parties. I've been ringing round their offices and have found them all a-flutter. Normally shunned into irrelevance by their small stature in parliament, it's on these rare occasions when their voting intentions really matter.

Most of the small-fry will be backing Labour: the Greens' Caroline Lucas, for example, and the SDLP's three MPs (although only two of them might actually make it to the Commons chamber). The Democratic Unionist party (eight MPs) and Plaid Cymru (three MPs) are still frantically trying to make up their minds on which way to go. "Give us a chance!" one harassed party official told me. The DUP is likely to back the Conservatives, however, taking Cameron's total to 311. If Plaid and the independents go with Labour, we're left with a 320-311 standoff going in Miliband and Clegg's favour.

That leaves the Scottish Nationalist party's six MPs. Initially a spokesperson made clear they were planning on not voting. But then came a clarification. They couldn't confirm that they weren't going to vote, after all. Instead we were provided with some thoroughly obtuse lines from Pete Wishart. "Our approach to the Leveson inquiry has been consensual and we will continue to work in this spirit and follow developments with interest," he explained, not explaining anything at all.

There's half a chance the SNP could abstain to draw attention to their separatist agenda - in this case manifesting itself in the form of Lord McCluskey, the former solicitor general who is carrying out a review of the Leveson recommendations north of the border. Wishart adds: "We can't prejudge what McCluskey's recommendations will be but will be looking at what is happening here."

Once the minority parties work out their positions we're likely to see Labour and the Lib Dems with their noses in front. Their majority doesn't look like getting out of single figures, though, which makes the inevitable question-marks about the solidity of the Labour and Conservative MPs' whipped votes critical.

Both sides have their dissenters. Miliband has a handful of Labour backbenchers - Frank Field and David Blunkett spring to mind - who have made clear they're not happy with statutory regulation. Cameron is likely to have a bigger headache when it comes to party discipline, though. The 'statutories', as they're known, led by Cameron's former spin doctor George Eustice, are likely to run into the tens. Around 50 Conservative MPs have expressed reservations with Cameron's ideas in the past, but many will be won over by the PM's reassurances. "We have a workable system ready to go," Cameron insisted this morning. Don't underestimate the ability of No 10 to win over waverers in the 100-odd hours between now and voting time.

The Hacked Off campaign group privately believes the vote is going to be very, very tight indeed. Both the minority party dynamic and the extent to which Tory and Labour party discipline holds up are being viewed as critical. Their assessment of both is that it's Cameron who has the most to worry about: he is likely to suffer more rebels, and is likely to struggle to win over the 'other' MPs beyond the DUP. A royal charter supported by primary legislation seems the most likely outcome - but this one is far, far too close to call.

All of which serves to explain why, for what I think is the first time, this prime minister - who has been so keen to ignore the realities of his failure to win an overall majority - has started talking about a "hung parliament". Leveson has forced him into an open acknowledgement that he simply does not have the default majority enjoyed by most of his predecessors in No 10. Cameron's rhetoric of a 'strong stable government' no longer holds up across the board.

He has taken to talking of the introduction of statutory underpinning of the press' self-regulation as a "rubicon" moment. It may well be. But the first time he talks of a "hung parliament" is a rubicon moment of its own. The coalition continues to exist, but it will not feel very much like it on Monday night.